When two leading members of the President's own political party join opposition party leaders and the NATO allies in urging a new departure in East-West relations - something new has been added which could make a difference.

This was the week when Howard Baker, who leads the Republicans in the United States Senate, and Charles Percy, who is Republican chairman of the Senate's powerful Committee on Foreign Relations, went together to the White House and personally urged President Ronald Reagan to open a dialogue about weapons with the Soviet Union.

There was a sense of urgency about their unusual action in breaking with the President openly and publicly on the issue of East-West strategy. They want action now. They want it partly because they think the President's strategy could prove damaging to Republicans during the election campaign just ahead. They want it also because Americans and Soviets are on the threshold of a race for weapons in space.

The President's strategy is to build first and then talk later when the US, in theory, will have a bargaining advantage.

The President wants to press ahead with an intensive research and development program for weapons which he hopes would put a protective umbrella over the US in outer space and make the US immune to an enemy's long-range nuclear weapons.

But is there time for such a strategy? Would it work? Might the Soviets forge ahead in the race for space as fast as can the US? Can the US afford the cost of such a defense? Should the US alter its nuclear strategy from ''mutual assured destruction'' to one of large-scale defense? And would the Soviets launch a preemptive strike if they thought the US was about to complete an effective defensive system?

No one can be sure of the answers. President Reagan has been persuaded that the US can gain a technical lead over the Soviets in such a race. He wants to spend about $26 billion over the next five years in seeking that advantage. He wants no restraints on his building program while the research and development continues.

This week he was encouraged in his position by the first successful interception of an intercontinental ballistic missile in outer space. ''Interception'' did work, far up in space, for the first time. It can be done. With enough ''interceptors'' the US might develop nuclear invulnerability.

But what will the Soviets be doing in the meantime, and what effect will it have on East-West relations? That was the question which caused allied leaders, led by Canadian Prime Minister Pierre Elliott Trudeau, to push Mr. Reagan in London last week to seek talks with the Soviets. And it was what caused Senators Baker and Percy to propose to the President in Washington this week that he institutionalize talks with the Soviets on a regular basis.

The White House argued that such meetings must be prepared in advance to be productive. Mr. Reagan does not want to go to any ''summit'' unless he can know in advance what will be the results.

Senator Baker, speaking on the White House lawn immediately after making his case to the President, called the President's argument ''anachronistic'' and added:

''Let's get together and talk about the general world situation, 'cause we've got to figure out some way not to blow each other up.''

The question now is whether the President will give in to the mounting pressure upon him and seek a ''summit'' with the Soviets. He gave no sign at the White House this week of having been deflected from his own preferred strategy by the two senators. The logic of the situation is that he will continue to seek from Congress the funds he wants for weapons development, and without strings attached, as long as possible.

White House spokesmen insist that Mr. Reagan sees himself as a ''peacemaker'' and that his goal is negotiation and ultimate agreement with the Soviets, but in his own time.

His immediate problem is that all of his international allies and now some of his most important domestic political allies are united in arguing that to wait longer is too dangerous. The Soviets have said that they are ready for a ban on antisatellite and other space weapons. Why not pick them up on that, go to a summit, and then try to talk there about other things which interest the US more?

Perhaps the first successful ''interception'' of an ICBM in flight can make a difference. If the ''interception'' is something beyond present Soviet capability and if, therefore, it worries the Kremlin, perhaps the Soviets might find the idea of a summit interesting.

And if they were to convey to Mr. Reagan privately a readiness for a summit, he might well decide that he too would like to have a talk. For Mr. Reagan to do a quick change of strategy on East-West talks would be no more surprising than when we learned that Richard Nixon had decided to visit Peking, and would be going.

Such a volte-face could become the sensation of the campaign year. It would defuse the East-West issue in the campaign and deprive the Democrats of one of their more promising pieces of political ammunition.

The week also produced a new development in the Middle East war. Iraq and Iran agreed to a mutual ban on shelling each other's cities. Whether the truce in such warfare against civilians will hold is questionable. But at this writing it was holding, and the long-expected Iranian offensive had not materialized. Observers began to speculate that, just perhaps, the Iranians know now that they cannot win a decisive victory, hence are getting positioned to accept a gradual winding down of the war.